Problem: 9 folders cost $16.56. Which equation would help determine the cost of 5 folders?
There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 5 folders. Since 5 folders cost $x$ , we have the following proportion: $\dfrac{5}{x}$ We can write the fact that 9 folders cost $16.56 as a proportion: $\dfrac{9}{\$16.56}$ The cost changes along with the number of folders purchased, and so the two proportions are equivalent. $\dfrac{5}{x} = \dfrac{9}{\$16.56}$